Field of the invention
This invention relates to a semiconductor laser device and particularly, but not exclusively, to a semiconductor laser device that emits visible radiation in the wavelength range 630 nm to 680 nm. The laser device may be of the edge-emitting or of the surface-emitting type.
Laser devices or laser diodes (LDs) fabricated in the (Al,Ga,In)P material system which emit visible light in the 630 nm-680 nm wavelength range are becoming increasingly important components of professional and consumer products. For example, it is envisaged that the Digital Video Disc (DVD) system will employ a 635 nm-650 nm wavelength LD capable of delivering up to 30 mW output power up to a temperature of 60xc2x0 C. The next generation of semiconductor lasers will need an even greater maximum power output up to a higher (eg. 70xc2x0 C.) operating temperature.
By the (Al,Ga,In)P system is meant the family of compounds having the general formula (AlyGa1xe2x88x92y)1xe2x88x92x InxP, where both x and y are between 0 and 1. One particular advantage of this semiconductor system is that it in lattice-matched to a GaAs substrate when the indium mole fraction, x, is equal to 0.48.
For convenience, the compound (AlyGa1xe2x88x92y)1xe2x88x92xInxP with x, yxe2x89xa00 and x, yxe2x89xa01 will generally be referred to an AlGaInP in the specification and claims. Similarly, the compound with y=1 will generally be referred to as AlInP, and the compound with yxe2x89xa00 will generally be referred to as GaInP.
2. Description of the Related Art
A principal limitation of current (Al,Ga,In)P laser diodes is that they are incapable of operating for long periods (or with a sufficiently low threshold current) at the highest specified operating temperature. It is generally believed that this is caused by electron leakage from the active region of the device into the surrounding optical guiding region and subsequently into the p-type cladding region.
The generic structure of a separate confinement laser structure intended to generate light at 630-680 nm will now be described with reference to FIGS. 1 and 2.
Curve (a) of FIG. 1 illustrates the difference between the xcex93-conduction band energy of (AlyGa1xe2x88x92y)0.52In0.48P, and Ga0.52In0.48P, as a function of the aluminum hole fraction in the quaternary alloy. Curves (b) and (a) of FIG. 1 show the difference between the X-conduction band energy and the xcex93-valance band energy respectively. FIG. 1 assumes that the bandgap difference between AlGaInP and GaInP is split in a ratio of 70:30 between the conduction band offset and the valance band off setxe2x80x94see S. P. Najda et al, xe2x80x9cJournal of Applied Physicsxe2x80x9d, Vol 77, No. 7, page 3412, 1995.
It will be soon that the minimum energy in the conduction band of (Al,Ga,In)P is a function of the aluminum content. There is a crossover from a xcex93-band minimum to an X-band minimum at an aluminum concentration of about 0.55.
The terms xcex93-band and X-band as used herein refer to symmetry points in the Brillouin zone and are standard terms in solid state physics, see for example R. A. Smith xe2x80x9cSemiconductorsxe2x80x9d, (Cambridge University Press, 1978). The terms xcex93-minimum and X-minimum refer to the minimum energy level of the xcex93-band and the X-band, respectively.
FIG. 2 is a schematic band structure of a separate confinement laser structure fabricated in the (Al,Ga,In)P system. It consists of an n-doped (Al0.7Ga0.3)0.52In0.48P cladding region 1, an (Al0.5Ga0.5)0.52In0.48P optical guiding region 2, 4, a GaInP quantum well active region 3 disposed within the (Al0.5Ga0.5)0.52In0.48P optical guiding region, and a p-doped (Al0.7Ga0.3)0.52In0.48P cladding region. Optical transitions giving rise to laser action in the quantum well active region 3 of the laser diode originate from xcex93-electrons in the GaInP quantum well active region.
The electron leakage current consists of that fraction of the electrons which have sufficient thermal energy to surmount the potential barrier on the right hand side of FIG. 2, and pass into the p-doped cladding region 5. It will be seen that xcex93-electrons are confined in the optical guiding region (waveguide region) by a potential barrier of only around 90 meV at the interface with the p-doped cladding region. This relatively small barrier height allows a significant proportion of electrons to escape. Moreover, holes in the valence band are confined only by a potential barrier of around 50 meV, and this low barrier height also allows significant carrier escape. Furthermore the X-conduction band in the p-cladding region 5 is some 50 meV below the xcex93-conduction band in the waveguiding region 2, 4, and this allows electrons to escape from the waveguiding region 2, 4 through the X-states in the p-doped cladding regions. Thus, the laser illustrated in FIG. 2 has a high leakage current, and so has poor performance at high temperatures.
In the laser structure shown in FIG. 2, the free carrier population of the n-doped cladding region 1 tends to saturate. This saturation of the free carrier concentration arises owing to the Fermi energy being pinned at the free energy level of the DX level in the n-doped cladding region 1. As is shown in FIG. 2, the DX level is lower than the X-band conduction minimum in the n-doped cladding region 1 (by around 100 meV). Saturation of the carrier concentration owing to Fermi pinning at the DX level in AlGaAs has been reported by A. Y. Du et al in xe2x80x9cApplied Physics Lettersxe2x80x9d Vol 66 No. 11, pages 1391-1393 (1995). Saturation of the carrier concentration in the (Al,Ga,In)P material system has been reported by S. P. Najda et al in xe2x80x9cJournal of Applied Physicsxe2x80x9d Vol. 82 No. 9, p. 4408 (1997).
Saturation of the free carrier concentration limits the number of electrons that can be injected into the active region of the laser device owing to carrier trapping at the DX level. This may lead to a charge imbalance, thereby reducing the efficiency of the laser device. Furthermore, saturation of the free carrier concentration in the n-doped cladding region 1 means that the electrical resistivity of the laser device cannot be reduced below a certain level.
This saturation of the carrier concentration is demonstrated in FIG. 8, which is taken from S. P. Najda et al (above). This figure shows experimental data on the silicon impurity concentration in (Al0.7Ga0.3)0.52In0.48P as a function of the temperature of the silicon cell. This Figure shows both the Hall carrier concentration (which is a measure of the free carrier concentration) and CV data (which is a measure of the fixed impurity concentration). It will be noted that the free carrier concentration (that is, the Hall carrier concentration) saturates at a concentration of n=4.8xc3x971017 cmxe2x88x923. This saturation is due to pinning of the Fermi level at the DX level.
The potential barrier between the optical guiding region 2, 4 and the p-doped cladding region 5 in the laser structure of FIG. 2 is only around 50 mV. Because this potential difference is small, there is a significant leakage of carriers from the optical guiding region into the p-doped cladding region. It is therefore desirable to increase the potential barrier between the optical guiding region 2, 4 and the p-doped cladding region 5.
FIG. 3 shown one way in which the potential barrier between the optical guiding region 2, 4 and the p-doped cladding region 5 can be increased. In the laser structure of FIG. 3, the n-doped cladding region 1 and the p-doped cladding region 5 have a greater aluminum composition than the cladding regions 1, 5 of the laser structure shown in FIG. 2. The cladding regions 1, 5 in FIG. 3 are formed of Al0.52In0.48P. It will be seen that there is now a potential barrier of around 250 meV to the transport of xcex93-electrons from the optical guiding region 2, 4 into the p-doped cladding region 5.
The use of an Al0.52In0.48P cladding region in FIG. 3 has the disadvantage that the free carrier concentration is expected to saturate at e value lower than the saturation value in (Al0.7Ga0.3)0.52 In0.48P. The cladding regions 1, 5 in the laser restructure of FIG. 3 will therefore have a very high resistivity, and a significant amount of heat will be generated by resistive heating in the cladding regions when the device is in use. Such heat generation is undesirable.
When the laser devices of FIGS. 2 and 3 are in use, electrons are injected from the n-doped cladding region 1 into the optical guiding region 2, 4. For efficient lasing to occur, the electrons should be able to enter the quantum well active region 3 and reach thermal equilibrium. However, the electron distribution associated with electron injection into the optical guiding region 2, 4 has a non-thermal distribution, and this results in non-linear optical gain and hot carrier effects. P. Bhattacharya et al. xe2x80x9cIEEE Journal of Quantum Electronicsxe2x80x9d Vol 32, No. 9, pages 1620-1628 (1996) propose that the problems associated with injecting carriers into the optical guiding region can be overcome by providing a thin potential barrier in the optical guiding region. Electrons that are injected into the optical guiding region reach the active region by tunnelling through the potential barrier, and this xe2x80x9ccoolsxe2x80x9d the electrons.
One disadvantage of the proposal by Bhattacharya et al (above) is that the thin potential barrier must be formed of a material that has a high aluminum content such as, for example, AlInP. A potential barrier formed of a material with a high aluminum content will create a quantum wall in the conduction band for X-electrons. Although electrons will be able to tunnel through the potential barrier, the X-electrons will be trapped in the quantum well formed in the potential barrier, and so will not be available for lasing in the active region. A further disadvantage of the proposal is that it results in a device in which a layer having a high aluminum content is disposed on one side of the active region and moreover is in the high gain part of the optical field. The refractive index of the potential barrier will be significantly different from the refractive index of the optical guiding region, owing to the large difference in their aluminum contents. Thus, the far-field image from such a device can be expected to be much more asymmetric than for the generic laser structure shown in FIG. 2.
A further problem with conventional edge-emitting lasers is that they have highly asymmetric far-field patterns. In many applications of edge-emitting lasers it is desirable for the far-field image to be substantially circular. For example, when an edge-emitting laser is used in a compact disc player, or when the output from an edge-emitting laser is coupled into an optical fibre, a substantially circular far-field profile is required. In order to use a conventional laser having an elliptical far-field profile in these applications it is necessary to provide correction optics. Typically, the asymmetric beam emitted by the laser is collimated, and is then passed through a pair of anamorphic prisms in order to obtain a near-circular far-field beam profile. The use of such correction optics increases the physical size, the complexity and the manufacturing cost of the system.
There have been a number of attempts to improve the aspect ratio of the beam emitted by an edge-emitting laser. S-T Yen et al, xe2x80x9cIEEB Journal of Quantum Electronicsxe2x80x9d Vol 32, No. 9, pages 1588-1595 (1996) and G. Lin et al, xe2x80x9cIEEE Photonico Technology Lettersxe2x80x9d Vol 8, No. 12, pages 1588-1590 (1996) have reported an InGaAs-AlGaAs laser having a vertical far-field of 13xc2x0 and a lateral far-field of 8xc2x0 , thus giving an: aspect ratio of approximately 1.6:1. (In comparison, a more typical aspect ratio for conventional lasers is approximately 3:1.)
According to P. Bhattacharya et al (above) and S-T Yen et al (above), many attempts to reduce the vertical far-field pattern below 15xc2x0 will cause either side lobes in the far-field pattern or an increase in the threshold current of the laser.
A first aspect of the present invention provides a laser device comprising: an n-doped cladding region and a p-doped cladding region; an optical guiding region disposed between the n-doped cladding region and the p-doped cladding regions and an active region disposed within the optical guiding region;
wherein the laser device further comprises an optical confinement layer disposed between the active region and one of the cladding regions, the optical confinement region having a lower refractive index than the cladding region.
The provision of the optical confinement layer with a lower refractive index than the cladding regions will provide improved optical confinement and hence reduce the penetration of the optical field into the cladding region beyond the optical confinement region and concentrate the optical field in the active region. Reducing the extent to which the optical field penetrates the cladding region will reduce the absorption that occurs in the cladding region and will also increase the optical field in the active region. The combination of these two effects will reduce the threshold current of the laser device. Furthermore, reducing the penetration of the optical field into the cladding region will improve the circularity of the far-field image.
A further advantage accrues when the optical confinement layer is provided between the n-doped cladding region and the active region. In this case, when electrons are injected from the n-doped cladding region into the active region they will pass through the optical confinement region. In doing so, the electrons will be xe2x80x9ccooledxe2x80x9d, and this will reduce carrier heating effects.
The optical confinement layer may be disposed at the interface between the optical guiding region and one of the cladding region.
The xcex93-conduction band of the part of the one cladding region immediately adjacent the optical confinement layer may be substantially degenerate with the X-conduction band of the optical confinement layer.
The one cladding region may have a graded bandgap.
The composition of the one cladding region may be selected such that the energy of the DX level in the one cladding region is greater than the Fermi level in the one cladding region. This can be achieved by using a direct bandgap material to form the cladding region. The DX level will not be significantly populated in this case, so that carrier trapping in the DX level will be substantially reduced.
Alternatively, the DX level in the part of the one cladding region adjacent the optical confinement layer may be substantially degenerate with the X-conduction band in the optical confinement layer. This increases the probability that carriers trapped in the DX level will tunnel out of the DX level into the X-conduction band of the optical confinement layer, and so reduces the trapping of carriers in the DX level.
The energy of the DX level in the one cladding region may increase away from the optical confinement layer. Grading the energy of the DX level in this way also increases the probability that carriers trapped in the DX level will tunnel out of the DX level into the X-conduction band of the optical confinement layer, and so reduces the trapping of carriers in the DX level.
The optical confinement layer may be disposed on the p-side of the laser device and may be p-doped. Band bending will occur if the optical confinement layer is heavily p-doped, and this band bending will increase the potential barrier to the transport of xcex93-electrons from the active region into the p-doped cladding region. Doping an optical confinement layer on the p-side of the device to be p-type will also facilitate the transport of holes from the p-doped cladding region into the optical guiding region and thus into the active region.
The laser device may comprise a second optical confinement layer disposed between the active region and the other of the cladding regions. The second optical confinement layer may be disposed at the interface between the optical guiding region and the other of the cladding regions.
The laser device may be fabricated in the (Al,Ga,In)P system, with the one cladding region being formed of AlGaInP having an aluminum mole fraction y. (It lattice matching to a GaAs substrate were required, the cladding layer would preferably be (AlyGa1xe2x88x92y)0.52In0.48P.)
The value of y may decrease away from the optical confinement layer.
The or each optical confinement layer may be an AlGaInP layer having a higher aluminum mole fraction than the one cladding layer. Alternatively, the or each optical confinement layer may be an AlInP layer. AlInP has a low refractive index of 3.253 (M. Moser et al xe2x80x9cApplied Physics Lettersxe2x80x9d Vol 64, No. 2, p. 235 (1994)), and thus provides good confinement of the optical field.
The or each optical confinement layer may consist of oxidised AlInP. Oxidising AlInP decreases its refractive index, so that using an optical confinement layer of oxidised AlInP will provide a further improvement in optical confinement.
The value of y may be approximately 0.4 at the interface between the one cladding region and the optical confinement layer. The xcex93-band of the cladding layer at the interface with an AlInP optical confinement layer is then substantially degenerate with the X-band of the optical confinement layer.
Alternatively, y may be approximately 0.9 at the interface between the one cladding region and the optical confinement layer. The DX level of the cladding layer at the interface with an AlInP optical confinement layer is then substantially degenerate with the X-band of the optical confinement layer.
The thicknesses of the optical guiding region and the or each optical confinement layer may be selected such that the laser device emits, in use, light having a substantially circular far-field-profile. Light emitted form such as laser device can be coupled directly into an optical fibre or used in a CD player, and the need to use corrective optics is eliminated.
A second aspect of the present invention provides a semiconductor laser device comprising an n-doped cladding region and a p-doped cladding region; an optical guiding region disposed between the n-doped cladding region and the p-doped cladding region; and an active region disposed within the optical guiding region; wherein the composition of one of the cladding regions is selected such that the energy of the DX level in the one cladding region is greater than the Fermi level in the one cladding region. The DX level will not be significantly populated in this case, so that carrier trapping in the DX level will be substantially reduced. The one cladding region may have a direct bandgap. The laser device may be fabricated in the (Al,Ga,In)P alloy system and the one cladding region may be formed of AlGaInP having an aluminum mole fraction y, where y less than 0.55.
A third aspect of the present invention provides a semiconductor laser device comprising: an n-doped cladding region and a p-doped cladding region; an optical guiding region disposed between the n-doped cladding region and the p-doped cladding region and an active region disposed within the optical guiding region; wherein the energy of the DX level in one of the cladding region, increases away from the optical guiding region. Grading the energy of the DX level in this way increases tho probability that carriers trapped in the DX level will tunnel out of the DX level into the optical guiding region, and so reduces the trapping of carriers in the DX level.
The laser device may be fabricated in the (Al,Ga,In)P alloy system, wherein the one cladding region is formed of AlGaInP having an aluminum mole fraction y, and wherein y decreases away from the optical guiding region.
The laser device may emit light in the visible range. The laser device, in one embodiment, emits light in the range from about 630 nm to about 680 nm. The laser device, in one embodiment, emits light is in the range from about 635 nm to about 650 nm.
The laser device may have a symmetrical structure. The laser device, in one embodiment, has a circular or elliptical far-field profile.
The laser device, in one embodiment, comprises two optical guiding regions having the same thickness. The laser device, in one embodiment, comprises two optical guiding regions having the same composition.